Highly Physical Etch Resistive Photoresist Mask to Define Large Height Sub 30nm Via and Metal Hard Mask for MRAM Devices

ABSTRACT

A conductive via layer is deposited on a bottom electrode, then patterned and trimmed to form a sub 20 nm conductive via on the bottom electrode. The conductive via is encapsulated with a first dielectric layer, which is planarized to expose a top surface of the conductive via. A MTJ stack is deposited on the encapsulated conductive via wherein the MTJ stack comprises at least a pinned layer, a barrier layer, and a free layer. A top electrode layer is deposited on the MTJ stack and patterned and trimmed to form a sub 30 nm hard mask. The MTJ stack is etched using the hard mask to form an MTJ device and over etched into the encapsulation layer but not into the bottom electrode wherein metal re-deposition material is formed on sidewalls of the encapsulation layer underlying the MTJ device and not on sidewalls of a barrier layer of the MTJ device.

TECHNICAL FIELD

This application relates to the general field of magnetic tunneling junctions (MTJ) and, more particularly, to etching methods for forming MTJ structures.

BACKGROUND

Fabrication of magnetoresistive random-access memory (MRAM) devices normally involves a sequence of processing steps during which many layers of metals and dielectrics are deposited and then patterned to form a magnetoresistive stack as well as electrodes for electrical connections. To define the magnetic tunnel junctions (MTJ) in each MRAM device, precise patterning steps including photolithography and reactive ion etching (RIE), ion beam etching (IBE), or their combination are usually involved. During RIE, high energy ions remove materials vertically in those areas not masked by photoresist, separating one MTJ cell from another. However, the high energy ions can also react with the non-removed materials, oxygen, moisture and other chemicals laterally, causing sidewall damage and lowering device performance.

To solve this issue, pure physical etching techniques such as ion beam etching (IBE) have been applied to etch the MTJ stack. However, due to the non-volatile nature, IBE etched conductive materials in the MTJ and bottom electrode can be re-deposited into the tunnel barrier, resulting in shorted devices. One approach to solve this issue is to greatly etch the MTJ so that the re-deposition can be confined below the tunnel barrier, without creating a shorting path. However two prerequisites have to be fulfilled to perform it. The first prerequisite is that the MTJ has to be built on top of large height vias that are thinner than the MTJ so that an adequate over etch does not induce re-deposition from the bottom electrode, which is usually wider than the MTJ. The second is that the metal hard mask, which also serves as the MTJ's top electrode, has to be thick enough so that enough remains after it is greatly consumed by the non-selective physical over etch. These considerations challenge the photolithography process since very thick photoresist is required for both cases, the patterns of which can easily collapse, especially when the size goes down to sub 30 nm. A new approach is needed to fully utilize the benefits of this technique.

Several references teach over etching to form MTJ's, including U.S. Patent Applications 2018/0040668 (Park et al) and 2017/0125668 (Paranipe et al). Other references teach thin vias on wider metal layers, such as U.S. Pat. No. 8,324,698 (Zhong et al). All of these references are different from the present disclosure.

SUMMARY

It is an object of the present disclosure to provide an improved method of forming MTJ structures.

Yet another object of the present disclosure is to provide a method of forming MTJ devices using a physical over etch to avoid both chemical damage and physical shorts.

A further object of the present disclosure is to provide a method of forming MTJ devices using a physical over etch into a dielectric layer encapsulating a metal via on a bottom electrode to avoid both chemical damage and physical shorts.

In accordance with the objectives of the present disclosure, a method for etching a magnetic tunneling junction (MTJ) structure is achieved. A conductive via layer is deposited on a bottom electrode, then patterned and trimmed to form a sub 20 nm conductive via on the bottom electrode. The conductive via is encapsulated with a first dielectric layer, which is planarized to expose a top surface of the conductive via. A MTJ stack is deposited on the encapsulated conductive via wherein the MTJ stack comprises at least a pinned layer, a barrier layer on the pinned layer, and a free layer on the barrier layer. A top electrode layer is deposited on the MTJ stack. The top electrode layer is patterned and trimmed to form a sub 30 nm hard mask. The MTJ stack is etched using the hard mask to form an MTJ device and over etched into the encapsulation layer but not into the bottom electrode wherein metal re-deposition material is formed on sidewalls of the encapsulation layer underlying the MTJ device and not on sidewalls of a barrier layer of the MTJ device.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of this description, there is shown:

FIGS. 1 through 8 illustrate in cross-sectional representation steps in a preferred embodiment of the present disclosure.

DETAILED DESCRIPTION

In the process of the present disclosure, we have demonstrated that due to the ultra-high selectivity between photoresist and metal under physical etch, one can simply use a photoresist mask alone to define a sub 30 nm metal via and hard mask with height larger than 100 nm. These will greatly increase the process margin when an MTJ over etch is required to reduce the metal re-deposition and associated electrically shorted devices.

In a typical process, photoresist is used to define dielectric or dielectric/metal hybrid hard masks by chemical RIE, before etching the metal vias underneath. Chemical plasma species which etch the hard mask quickly but consume photoresist slowly can be used at this first step. Then, the metal via is defined using other chemical plasma species which etch the metal quickly but consume the hard mask slowly. By doing this, a larger height of metal vias can be obtained than by using photoresist alone. However, the photoresist is always consumed quickly under chemical RIE, no matter what chemistry is used, thus limiting the height of the metal via to lower than 50 nm when the pillar size goes down to −30 nm or below.

In the process of the present disclosure, the dielectric hard mask is not required because the photoresist is consumed very slowly due to photoresist's slow etch rate under physical etching. Using a photoresist mask alone, we apply a physical etch such as pure Ar RIE or IBE to pattern the vias and hard mask metals due to its better selectivity than a regular chemical RIE. Later, assisted by high angle IBE trimming, sub 20 nm vias and sub 30 nm metal hard masks with height larger than 100 nm are fabricated, making it possible to greatly over etch sub 30 nm MTJ's without chemical damage and re-deposition on the tunnel barrier.

Referring now to FIGS. 1 through 8, the novel method of the present disclosure will be described in detail. Referring now more particularly to FIG. 1, there is shown a bottom electrode 10 formed on a substrate, not shown. Now, a metal layer 12 for forming a via, such as Ta, TaN, Ti, TiN, W, Cu, Mg, Ru, Cr, Co, Fe, Ni, Pt, Ir, Mo, or their combinations such as TiN/Co/Ir, TiN/TiN/Ir, etc. with thickness h1 of 80-150 nm, and preferably ≥100 nm, is deposited onto the bottom electrode 10, which may be made of similar materials. An organic or dielectric anti-reflective coating 14 such as SiN, SiON, or SiCOH with thickness h2 of ≥20 nm is then deposited on the metal layer 12. Preferably a dielectric anti-reflactive coating is used since it reduces light reflection more effectively. Moreover, the dielectric anti-reflective coating can act a hard mask for etching, so no additional hard mask is required. Photoresist is spin-coated and patterned by 248 nm photolithography, forming patterns 16 with size d1 of −70-80 nm and height h3 of ≥200 nm.

The anti-reflective coating 14 and metal layer 12 are etched by a physical etch such as pure Ar RIE or IBE, as shown in FIG. 2. As illustrated in Table 1 below, if Ta is used for the via material, the etch rate ratio of photoresist to Ta is 1.6:1, smaller than that of 2.4:1 when CF₄ RIE is used. When the metal material is TiN, the etch rate ratio of photoresist to TiN is 1.7:1, much smaller than that of 4.5:1 using the same CF₄ RIE.

TABLE 1 Selectivity comparison between CF₄ RIE and Ar RIE/IBE. Selectivity Selectivity Plasma (Etch rate of (Etch rate of species PR:Ta) PR:TiN) CF₄ 2.4:1 4.5:1 Ar 1.6:1 1.7:1

In both cases, these relatively slow etch rates of photoresist compared to metals allow for the fabrication of metal pillars with height ≥100 nm and size d2 of ˜50-100 nm. The etch rate of photoresist is at most twice the etch rate of the underlying metal. Depending on the thickness of the metal 12, the anti-reflective coating 14 can be partially consumed. An anti-reflective coating height h4 of ≥15 nm remains.

Now, a high angle IBE trimming 20 with an angle Θ1 ranging from 70-90° with respect to the surface's normal line is applied to the metal pillar. The metal patterns shrink horizontally. The resulting via size d3 ranges from 10-20 nm, depending on the IBE trimming conditions such as RF power (500-1000 W) and time (100-1000 sec). Here, ex-situ IBE trimming is used when the metal vias are made of inert metals and in-situ IBE trimming is needed for metals that can be readily oxidized in air. Due to the protection of the remaining dielectric 14 on top and the extremely low vertical etch rate (≤5A/sec) of IBE at such a large angle, the remaining via's height h5 is the same as the as-deposited height h1 or decreases less than 5 nm after this step, as shown in FIG. 3.

Next, referring to FIG. 4, a dielectric material 22 such as SiO₂, SiN, SiON, SiC, or SiCN with thickness ≥100 nm is deposited to encapsulate the vias 12. Here, ex-situ encapsulation is used when the metal vias are made of inert metals and in-situ encapsulation is needed for metals that can be readily oxidized in air. Chemical mechanical polishing (CMP) is applied to smooth the surface as well as to expose the metal vias underneath, with remaining via height h6 of ≥80 nm. One should note that these dielectric surrounded small size vias make the later MTJ over etch possible without etching the bottom electrode.

Now, layers are deposited on encapsulated vias to form magnetic tunnel junctions. For example, pinned layer 24, tunnel barrier layer 26, and free layer 28 are deposited. There may be one or more pinned, barrier, and/or free layers. The MTJ layers have a height h8 of 10-30 nm. A top electrode 30 made of similar material to the vias with thickness h9 of ≥100 nm is deposited on the MTJ layers. A second anti-reflective coating 32 is deposited on the top electrode 20 followed by photoresist coating, exposure, and development to form photoresist patterns 34 with size d1 of ˜70-80 nm and height h3 of 200 nm.

The top electrode 30 is physically etched and IBE trimmed, as shown in FIG. 6. The hard mask etch and trim can use the same conditions such as gas species/RF and bias powers/angles as the earlier metal via etch. However, it should be noted that in order to trim or optimize the hard mask's roughness to improve the uniformity of feature size and device performance, one can use different etch conditions. The resulting metal hard mask/top electrode 30 has a size d4 of ˜30 nm and height h10 of ≥100 nm.

Now, the MTJ stack 24/26/28 can be etched by the same type of physical etch as the hard mask with the same or different etch conditions. The MTJ stack can also be etched by chemical RIE or a combination of physical RIE, IBE and chemical RIE. After a great physical over etch of the MTJ, far into the dielectric encapsulation layer 22, but not all the way down to the underlying bottom electrode 10, all the metal re-deposition 36 is confined below the tunnel barrier layer 26. Furthermore, there is no metal re-deposition from the bottom electrode 10 as it remains covered by the dielectric encapsulation 22, as shown in FIG. 7. Also, the sub 20 nm vias 12 underneath the MTJ stacks are not etched since the via width is smaller than the MTJ width. Importantly, the remaining metal hard mask 30 has a remaining height h11 of larger than 50 nm. This leaves enough process margin for the following steps.

As shown in FIG. 8, a second dielectric layer 38 is deposited to encapsulate the vias 12, MTJ structure 24/26/28, and top electrode 30. CMP and surface sputtering are applied to smooth the surface as well as to expose the top electrode followed by top metal deposition 40.

In summary, the process of the present disclosure uses a photoresist mask alone to physically etch the via and hard mask metals below and above the MTJ. Due to better selectivity of this type of etch as compared with a regular RIE etch, we can achieve sub 60 nm metal hard mask and vias with height larger than 100 nm. Later, assisted by high angle IBE trimming, we can decrease the feature size to sub 30 nm, without significantly reducing the via height. This will greatly increase the process margin as well as reduce the metal re-deposition through an MTJ over etch. It should be noted that this is a low cost technique, since we achieve this without involving expensive immersion 193 nm or EUV photolithography or a complex hard mask stack. The process of the present disclosure will be used for MRAM chips of the size smaller than 60 nm as problems associated with chemically damaged sidewall and re-deposition from bottom electrode become very severe for the smaller sized MRAM chips.

Although the preferred embodiment of the present disclosure has been illustrated, and that form has been described in detail, it will be readily understood by those skilled in the art that various modifications may be made therein without departing from the spirit of the disclosure or from the scope of the appended claims. 

What is claimed is:
 1. A method for fabricating a magnetic tunneling junction (MTJ) structure comprising: depositing a conductive via layer on a bottom electrode; patterning said conductive via layer and then trimming said conductive layer to form a sub 20 nm conductive via on said bottom electrode; encapsulating said conductive via with a first dielectric layer and planarizing said first dielectric layer to expose a top surface of said conductive via; thereafter depositing a MTJ stack on encapsulated said conductive via wherein said MTJ stack comprises at least a pinned layer, a barrier layer on said pinned layer, and a free layer on said barrier layer; depositing a top electrode layer on said MTJ stack; patterning said top electrode layer and then trimming said top electrode layer to form a sub 30 nm hard mask; and thereafter etching said MTJ stack using said hard mask to form an MTJ device and over etching said MTJ stack into said encapsulation layer but not into said bottom electrode wherein metal re-deposition material is formed on sidewalls of said encapsulation layer underlying said MTJ device and not on sidewalls of a barrier layer of said MTJ device.
 2. The method according to claim 1 wherein said bottom electrode, said conductive via layer, and said top electrode comprise Ta, TaN, Ti, TiN, W, Cu, Mg, Ru, Cr, Co, Fe, Ni, Pt, Ir, Mo, or their combinations with height of 80-150 nm, and preferably 100 nm.
 3. The method according to claim 1 wherein said patterning said conductive via layer comprises: depositing an organic or dielectric anti-reflective coating comprising SiN, SiON, or SiCOH to a thickness of ≥20 nm on said conductive via layer; spin-coating a photoresist layer on said anti-reflective coating; patterning said photoresist layer to form photoresist patterns with width of 70-80 nm and height of ≥200 nm; and physically etching said anti-reflective coating and said conductive via layer with IBE or RIE with pure Ar plasma to form metal pillars with height ≥100 nm and width 50-100 nm.
 4. The method according to claim 1 wherein said trimming said conductive via layer comprises in-situ or ex-situ IBE trimming at an angle of 70-90° with respect to a normal line of a top surface of said conductive via layer with radio frequency (RF) power of 500-1000 W for 100-500 seconds wherein a pattern size of trimmed said conductive via is 10-20 nm.
 5. The method according to claim 4 wherein a vertical etch rate of said IBE trimming is ≤5A/sec wherein a height of said conductive via is the same as a height of as-deposited said conductive via layer or said height of said conductive via decreases by less than 5 nm from said as-deposited height.
 6. The method according to claim 1 wherein said encapsulating said conductive via with said first dielectric layer comprises in-situ or ex-situ depositing SiO₂, SiN, SiON, SiC, or SiCN having a thickness of ≤100 nm.
 7. The method according to claim 1 wherein said patterning said top electrode layer comprises: depositing an organic or dielectric anti-reflective coating comprising SiN, SiON, or SiCOH to a thickness of ≥20 nm on said top electrode layer; spin-coating a photoresist layer on said anti-reflective coating; patterning said photoresist layer to form photoresist patterns with size of 70-80 nm and height of ≥200 nm; and physically etching said anti-reflective coating and said top electrode layer with IBE or RIE using pure Ar plasma to form said sub 30 nm hard mask with height ≥100 nm.
 8. The method according to claim 1 wherein said etching and said over etching said MTJ stack comprises physical etching with IBE or RIE with pure Ar, chemical RIE, or a combination of physical RIE, IBE and chemical RIE.
 9. The method according to claim 1 wherein said top electrode has an as-deposited height of ≥100 nm and wherein after said etching and over etching of said MTJ stack, a height of said top electrode is ≥50 nm.
 10. The method according to claim 1 after said etching and over etching of said MTJ stack further comprising: encapsulating said MTJ device with a second dielectric layer; smoothing a top surface of said second dielectric layer and exposing a top surface of said top electrode; and thereafter depositing a top metal layer contacting said top electrode.
 11. A method for fabricating a magnetic tunneling junction (MTJ) structure comprising: depositing a conductive via layer on a bottom electrode; patterning said conductive via layer and then trimming said conductive layer to form a sub 20 nm conductive via having a height 50 nm on said bottom electrode; encapsulating said conductive via with a first dielectric layer and planarizing said first dielectric layer to expose a top surface of said conductive via; thereafter depositing a MTJ stack on encapsulated said conductive via wherein said MTJ stack comprises at least a pinned layer, a barrier layer on said pinned layer, and a free layer on said barrier layer; depositing a top electrode layer on said MTJ stack; patterning said top electrode layer and then trimming said top electrode layer to form a sub 30 nm hard mask; and thereafter etching said MTJ stack using said hard mask to form an MTJ device and over etching said MTJ stack into said encapsulation layer but not into said bottom electrode wherein metal re-deposition material is formed on sidewalls of said encapsulation layer underlying said MTJ device and not on sidewalls of a barrier layer of said MTJ device.
 12. The method according to claim 11 wherein said bottom electrode, said conductive via layer, and said top electrode comprise Ta, TaN, Ti, TiN, W, Cu, Mg, Ru, Cr, Co, Fe, Ni, Pt, Ir, Mo, or their combinations with height of 80-150nm, and preferably ≥100 nm.
 13. The method according to claim 11 wherein said patterning said conductive via layer comprises: depositing an organic or dielectric anti-reflective coating comprising SiN, SiON, or SiCOH to a thickness of ≥20 nm on said conductive via layer; spin-coating a photoresist layer on said anti-reflective coating; patterning said photoresist layer to form photoresist patterns with width of 70-80 nm and height of ≥200 nm; and physically etching said anti-reflective coating and said conductive via layer with IBE or RIE with pure Ar plasma to form metal pillars with height ≥100 nm and width 50-100 nm.
 14. The method according to claim 11 wherein said trimming said conductive via layer comprises in-situ or ex-situ IBE trimming at an angle of 70-90° with respect to a normal line of a top surface of said conductive via layer with radio frequency (RF) power of 500-1000 W for 100-500 seconds wherein a pattern size of trimmed said conductive via is 10-20 nm.
 15. The method according to claim 14 wherein a vertical etch rate of said IBE trimming is ≤5A/sec wherein a height of said conductive via is the same as a height of as-deposited said conductive via layer or said height of said conductive via decreases by less than 5 nm from said as-deposited height.
 16. The method according to claim 11 wherein said encapsulating said conductive via with said first dielectric layer comprises in-situ or ex-situ depositing SiO₂, SiN, SiON, SiC, or SiCN having a thickness of ≥100 nm.
 17. The method according to claim 11 wherein said patterning said top electrode layer comprises: depositing an organic or dielectric anti-reflective coating comprising SiN, SiON, or SiCOH to a thickness of ≥20 nm on said top electrode layer; spin-coating a photoresist layer on said anti-reflective coating; patterning said photoresist layer to form photoresist patterns with size of 70-80 nm and height of ≥200 nm; and physically etching said anti-reflective coating and said top electrode layer with IBE or RIE using pure Ar plasma to form said sub 30 nm hard mask with height ≥100 nm.
 18. The method according to claim 11 wherein said etching and said over etching said MTJ stack comprises physical etching with IBE or RIE with pure Ar, chemical RIE, or a combination of physical RIE, IBE and chemical RIE.
 19. The method according to claim 11 wherein said top electrode has an as-deposited height of ≥100 nm and wherein after said etching and over etching of said MTJ stack, a height of said top electrode is ≥50 nm.
 20. A magnetic tunneling junction (MTJ) comprising: a sub-30 nm MTJ device on a sub-20 nm conductive via encapsulated by a dielectric layer; a bottom electrode underlying said conductive via; and a top electrode overlying and contacting said MTJ device. 